1. Field of the Invention
The present invention relates to a wavelength division multiplexing network, and relates in particular to a communication network where multiplexed signal transmission lines are connected in a ring through a plurality of nodes that can be used to process multiple wavelengths. This technology enables to reduce the number of hardwares needed inside the node and simplify the system configuration, and enables to switch optical paths to bypass a fault, thereby enabling to continue operating the network even when the failure is within the node. The technology is particularly useful when a two-fiber bi-directional ring network has been serviced to its capacity, which can be increased by transforming the network into a 4-fiber directional ring network, without stopping the normal operation of the existing network.
2. Description of the Related Art
FIG. 15 is a schematic diagram of a WDM ring network, which is an example of the conventional wavelength multiplexing optical communication network. The WDM ring network is comprised by: nodes 901axcx9c901e; optical fibers 902 serving as WDM transmission lines, an optical path 903a for providing normal optical communication through the optical fiber 902, and an optical path 903b contained in the optical fiber 902, which is used when problems develop in the optical path provided in the optical fiber 902. Here, the logical connection between each node is conducted using wavelengths as routing information, and these signal channels are called optical paths.
During the normal communication in this WDM ring network, WDM signals are input in the optical path 903a. In other words, optical signals input in node 901a are output from node 901c by propagating clockwise by way of node 901b. 
Suppose that a fault 904 develops between the nodes 901a and 901b in the optical path 901, as shown in FIG. 16, signals cannot be propagated between the nodes 901a, 901b. Therefore, WDM signals entering node 901a are first propagated counter-clockwise through the nodes 901e, 901d, 901c and 901b, and are then propagated clockwise in the optical path 903b through the nodes 901b, 901c to be output from the node 901b. 
FIG. 17 is a schematic diagram of an example of the general configuration of the WDM optical communication network, in which the node structure of a two-fiber unidirectional ring, that allows extraction/insertion (adding/dropping) of any wavelength, is applied to a two-fiber bi-directional ring.
This type of WDM optical communication network is reported, for example, in L. Berthelon et. al., Proc. GLOBECOM 96, pp. 311-315, 1996, or A Mariconda et. al., Proc. ECOC 96, ThD. Jan. 10, 1996. These articles describe a general structure for the application of the node structure of a two-fiber unidirectional ring, that allows processing of any wavelength, to a two-fiber bi-directional ring.
This type of WDM optical communication network is operated using single wavelength 2xc3x972 optical switches that may include wavelength filters to enable extraction or insertion of waves, and the spectral source for different wavelengths is a fixed-wavelength source, and the system does not include a device for wavelength conversion. Also, in general, switching of optical path during circuit problems is considered in such ring networks, but in this discussion, switching is not considered for simplification. An example of switching is described later in Conventional Technology 2.
Node B (1000) in such a WDM optical network is connected to two adjacent nodes A and B having the same structure as the node B through optical fibers 911xcx9c914, and supplies M-channels (or channels) of a required wavelength to the optical paths in a full mesh configuration between the nodes. This WDM network is comprised by: optical add/drop circuits 1001, 1002 for processing at least Nxe2x88x921 waves of a given wavelength; and the optical add/drop circuits 1001, 1002 are provided with wavelength de-multiplexers 1003, 1004 for de-multiplexing M input waves of WDM signals; 2xc3x972 optical switches 10051 to 1005M/2; and optical couplers (or wavelength multiplexers) 1007, 1008.
Also, this WDM network is provided with optical path (op) termination circuits (transmit end and receive end) for selecting the optical paths, and the transmit end 1009 of the op termination circuit is provided with M pieces of fixed-wavelength light source 10101xcx9c1010M; M pieces of modulators 10111xcx9c1101M for superimposing electrical signals on optical signals; and M lines of electrical input 10121xcx9c1012M and the receive end 1013 of the op termination circuit is provided with M lines of photo-electric converter 10141xcx9c1014M for converting optical signals of respective wavelengths to electrical signals; and M lines of electrical signal output 10151xcx9c1015M.
Here, optical fiber 911 contains optical signals input from node A, and optical fiber 912 contains optical signals input from other node C, and optical fiber 913 contains optical signals output to node C, and optical fiber 914 contains signals output to node A.
Bi-directional communication between node B and the other node is carried out in the following manner.
Here, the direction of nodes are defined such that A B C is clockwise (clock) and C B A is counter-clockwise. Also, for the counter-clockwise direction, the waves are used in the ascending order of refractive index stating from the lowest index using M/2 waves, and for the counter-clockwise direction, the waves are used in the descending order of refractive index starting from the highest index using M/2 waves. If the same wavelength is used in both directions, M/2 waves are sufficient number of waves required, but, for use in public networks, it is necessary to consider protection circuits, and in such cases, the remaining M/2 waves in each fiber is used generally for emergency use. Therefore, in this discussion, it is left as M-channels. Also, the reason for using different wavelengths for clockwise and counter-clockwise directions is to prevent wave collision for lines at the insertion circuit during switching operations, and this aspect of the circuit will be discussed later in the section related to Technology 2.
In a clock optical path from node B to another node, for example node C, one wave of the xcex1xcx9cxcexM/2 modulated by one of the electrical signal input 10121xcx9c1012M/2 is input in the optical insertion circuit 1001, and is output to optical fiber 913 through one of the optical switches 10051xcx9c1005M/2. On the other hand, an optical path from node C to node B is a counter-clockwise path, so that one wave of the xcexM/2+1xcx9cxcexM is allocated, and it is input in optical fiber 912 into node B, and is output to the receiving end 1013 of the op termination circuit through one of the optical switches 10061xcx9c1006M/2 in the optical add/drop circuit 1002.
In this type of WDM network, to enable insertion/extraction of any wavelength at a node, it is necessary to be able to process each M-channels in the optical add/drop multiplexing circuit (OADM), as well as to couple all the 2M-channels multiplexed by the two wavelength de-multiplexers to the op termination circuit. Therefore, in order to produce an optical path using any wavelength, it is necessary to provide a modulator in each of the transmit ends of the op termination circuit of all the 2M-channels, and in order to receive any wavelength of the 2M-channels, it is necessary for each of the receive ends of the op termination circuit to have an op termination circuit.
The above configuration has an advantage of offering logical connectivity between the nodes, that is, it does not restrict the traffic distribution pattern, however, assuming that the network is in a full mesh configuration, which is a typical logical connectivity between the nodes N, each node needs to process Nxe2x88x921 channels of the 2M-channels, so that the number of coupling lines between the OADMs and op termination circuits as well as the number of modulators and the op termination circuits are quite redundant compared with the necessary number of channels Nxe2x88x921.
Also, even if a ring network contains a large number of optical paths greater than the number corresponding to a full mesh configuration, the number of waves that each node needs to process is of the order of N, compared with the number of channels M, which is of the order of N2 in this case, so that the overall system design is highly redundant.
Also, to solve these problems, multiple of optical signal transmission lines containing individual channels branched in the OADM must be manually connected to the required number of op termination circuits, and the optical signal transmission lines containing individual channels to be inserted in the OADM must be manually connected to the required number of modulators, so that it has been difficult to process individual channels automatically.
FIG. 18 is a schematic diagram of another example of the conventional WDM network having switching functions, and includes: node B (1100) provided with a WDM network, optical fibers 911xcx9c914 connecting adjacent nodes A and C of the same structure as node B; 2xc3x972 optical switches 1103, 1104 for switching of WDM signals in units of M-channels between the fibers, and provide full mesh optical paths among the nodes for necessary M-channels.
This WDM network is provided with optical add/drop multiplexing circuits (OADM) 1101, 1102 for processing at least Nxe2x88x921 waves of any wavelength, and the circuits 1101, 1102 include: respective wavelength de-multiplexers 1105, 1106, 2xc3x972 optical switches 11071xcx9c1107M, 11081xcx9c1108M for processing one wavelength; and wave couplers (or wave multiplexers) 1109, 1110 for multiplexing M-channels.
This WDM network is operated by the op termination circuits 1111, 1116 for selecting optical paths, in which the transmit end 1111 has M pieces of fixed wavelength spectral source 11121xcx9c1112M each emitting different wavelengths, M pieces of 1:2 duplication circuits 11131xcx9c1113M for duplicating output signals from the fixed wavelength light source 11121xcx9c1112M; 2M pieces of modulators 11141xcx9c11142M for superimposing electrical signals on optical signals; and 2M lines of electrical signal input 11151xcx9c11152M. The receive end 1116 of the optical path (op) termination circuit has 2M pieces of photo-electric converters 11171xcx9c11172M and 2M lines of electrical signal output 11181xcx9c11182M.
Of the modulators 11141xcx9c11142M, modulators 1114M/2+1xcx9c11143M/2 are reserve (protection) modulators, and of the 2M pieces of photo-electric converters 11171xcx9c11172M and 1117M/2+1xcx9c11173M/2 are protection signal op termination circuits.
In this case, optical fiber 911 contains optical signals input from node A, optical fiber 912 inserts optical signals input from other node C, optical fiber 913 contains optical signals to be output to node C; and optical fiber 914 contains optical signals to be output to node A. The 2xc3x972 optical switches 1103, 1104 are arranged so that, when there is no circuit problems, optical signals input from node A are output to node C through the optical add/drop circuit 1101, and to transmit optical signals input from node C to node A through the OADM 1101.
When there is a problem, this WDM network is able to continue its operation without changing the wavelength in the faulty optical path using the two unidirectional lines having different allocated wavelengths between certain bi-directional lines.
In this example, signal transmission from node A to node B, that is, clockwise signals use xcex1xcx9cxcexM/2 during the normal operation while xcexM/2+1xcx9cxcexM are used during the problem period. Signal transmission from node B to node A, that is counter-clockwise signals use xcex1xcx9cxcexM/2 during the problem period, and during the normal operation, xcexM/2xe2x88x921xcx9cxcex1 are used. Here, xcex1 and xcexM/2+1 respectively are allocated to clockwise path and counter-clockwise path, and similarly, xcexM/2 and xcexM are allocated to clockwise path and counter-clockwise path, respectively, between the nodes. During the normal operation, two fibers both transmit M/2 channels of WDM signals.
Switching operation of the above WDM circuit will be illustrated with reference to FIGS. 16 and 18.
In this ring network, when a fault 904 develops, switching is based on changing an entire WDM section containing all the optical paths that include the faulty fiber having the fault 904 in the faulty line 904a, by isolating the end nodes 901a, 901b at the 2xc3x972 switches 1103, 1104 to switch the optical signals input in the problem node.
For example, if a fault develops between node B and node A, optical switch 1103 changes the optical path of output signals (xcexM/2+1xcx9cxcexM) from node C, input through the fiber 912, so as to input the signals in the OADM 1101. Therefore, the wavelengths (xcexM/2+1xcx9cxcexM) that should be terminated at node B are coupled, and the wavelengths (xcex1xcx9cxcexM) that should be inserted at node B is inserted, and are output to node C through the optical switch 1104. At this time, there is no need for the optical switch 1104 to change line.
Also, of the wavelength (xcex1xcx9cxcexM) to be inserted at OADM 1101, the wavelengths (xcexM/2+1xcx9cxcexM), that should have been modulated in the modulators 11143M/2+1xcx9c11142M, inserted in the OADM 1102, and output to optical fiber 914 through the optical switch 1104, are input in OADM 1101 by operating the reserve modulators 1114M/2+1xcx9c1114M. In the add/drop circuit 1101, the inserted wavelengths (xcex1xcx9cxcexM/2) do not collide with the new wavelengths to be inserted (xcexM/2+1xcx9cxcexM).
In the meantime, the wavelengths (xcex1xcx9cxcexM/2), input in node A and should have been coupled at node B, are switched and input in node C, and are converted to electrical signals in the reserve photo-electric converters 1117M+1xcx9c11173M/2 used protection.
According to the method of emergency operation in this WDM network, because network protection is based on looping back the signals in units of optical multiplexing section (OMS protection) between the nodes 901a, 901b, which are the nodes at both ends of the fault X, the number of sections between the nodes that are required to bypass the problem section are increased significantly. For example, the maximum number of sections is 3(Nxe2x88x921)/2 for an odd number of nodes N, and 3N/2xe2x88x921 for an even number of nodes N. It results in operational problems such as increased distance for optical paths, signal delays and requirement for increased number of repeater stations, resulting that it is difficult to design a large-scale network.
Also, in this WDM network, in order to prepare for equipment failures involving modulators and add/drop circuits, it is necessary to duplicate the number of devices to process individual waves in the op termination circuits, in addition to devices such as 2xc3x972 switches 1103, 1104 that are used during the emergency.
Further, because switching is based on units of WDM sections, even when only a part of the optical paths in a WDM section is faulty, it is necessary to switch the entire section including the normal unaffected optical paths.
Accordingly, conventional WDM network using conventional node apparatus presented the following problems in processing multiple waves having a plurality of wavelengths.
(1) When a single wavelength is used to transmit optical data between an optical add/drop circuit and an op termination circuit, the number of optical signal transmission lines required is equal to (M-channelsxc2x7number of fibers between nodes).
(2) M pieces of photo-electric converters are required in the receiving end of the op termination circuit in order to process Nxe2x88x921 waves of the M-channels.
(3) In order to solve these problems, it is necessary to manually connect the lines between the processing circuits and op termination circuits as well as between the light source and switches within the optical path termination circuit.
(4) Emergency switching operation is based on units of WDM signals propagating in multiple optical paths contained in one fiber, therefore, it is necessary to provide loop-back lines at both nodes surrounding a fault, thereby resulting in long length of optical paths and presenting a deterrent to designing a large-scale network.
(5) The 2xc3x972 switch for switching the WDM signals cannot continue to function when there is a fault within the node.
To resolve these problems, it is necessary to provide devices additional to the 2xc3x972 switch to perform switching for each wave.
Also, if such configuration is adopted, it is necessary to provide switches to change from the normal wavelengths to respective protection wavelengths for all M-channels for both transmit end and receive end of the op termination circuit.
It is an object of the present invention to provide an optical communication network, based on a wavelength division multiplexing (WDM) method, in which the optical paths are connected by the nodes in a ring architecture, which enables to process any number of waves through the nodes, to reduce the requirement for the number of hardwares, and to simplify the structure of the network. In the present ring network, emergency operation is based on switching of optical paths for network protection so that problems within a node can also be resolved.
The object has been achieved in a wavelength division multiplexing network, based on a plurality of lines of optical fibers to connect a plurality of nodes into a ring network architecture, using a half of the fibers for operating in a clockwise direction and a remaining half of the fibers for operating in a counter-clockwise direction to form a logical network comprised by signal channels contained in individual fibers, wherein each node is comprised by a plurality of optical add/drop circuits; a transmit end which assigns waves to signal channels and sends them to the optical add/drop circuit, and a receive end which receives signal channels sent from the optical add/drop circuit; and transmission lines provided between said optical add/drop circuits and said transmit end, as well as between said optical add/drop circuits and said receive end for transmitting any multiple waves to be processed within said node so as to establish mutual communication by transmitting processed multiple waves to other nodes.
A first aspect of the network connected in a plurality of lines of optical fibers architecture is summarized as follows that the optical add/drop devices (OADM) are capable of processing many wavelengths, and WDM transmission lines carry optical signals between the OADMs and transmit/receive ends of the optical termination circuits that select the optical paths for respective multiplexed optical signals.
In a specific example, if it is supposed that the network is comprised by N nodes, and M-channels (waves) are required to connect the network in a full mesh configuration, in which the sending side of each node must be structured so as to able to select Nxe2x88x921 waves from the M-waves, and the selected waves are input in the OADM through the WDM transmission lines and are transmitted to the receiver side of each node, where the multiplexed wave signals are de-multiplexed and individual signals are transmitted to the receive end of the optical termination circuit.
A second aspect of the network is that the capability of selecting Nxe2x88x921 waves from M-channels is achieved by providing M number of fixed wave sender groups combined with Mxc3x97Mxe2x80x2 optical switches (where Mxe2x80x2=Nxe2x88x921 for example), or by providing a tunable wave sender capable of sending the same number of waves that are needed to be selected.
In this example, the network parameter M is computed from M=(N2xe2x88x921)/4 for two fiber network, or M=(N2xe2x88x921)/8 for four fiber network, therefore, it is recognized that the number of waves to be inserted in each node is less than the number of channels M required to provide a full mesh configuration, so that each node needs to process only the number of waves required for its own services. This approach reduces the amount of hardwares required within each node to match the number of waves to be processed in individual nodes.
A third aspect of the network is that the spectral source made up of M fixed wave optical sender groups and Mxc3x97Mxe2x80x2 optical switches may be replaced with a number of fixed wavelength sender groups and an optical switch of a small scale.
In this case, when the number of nodes N within the network is high, which means that the required number of channels M is also high, it is not necessary to use a large-scale switching device emitting a single wave so that a small scale switch is acceptable. The network structure is configured so that the network capacity can be increased by adding a required number of small-scale fixed wave senders according to the number of optical paths to be processed by individual nodes.
A fourth aspect of the network is that, when a fault is developed within a ring network, ring network operation can be continued by providing a switching capability to switch only those optical paths that are faulty.
In this case, each node is provided with an optical path protection capability to switch transmission of optical signals on the basis of the optical path, so that problems within the node can be resolved individually. Furthermore, compared with the problem in the conventional section-based protection, it is possible to prevent an increase in the line length for protection optical paths.
A fifth aspect of the network is that increased demand for network services is resolved by providing extra-traffic lines using protection waves, which are reserved for emergency use, for the normal communication services.
In this case, a wave emitted from an optical sender is duplicated, and one signal is transmitted through the optical path due to normal information, and other signal is transmitted through a new optical path due to extra-traffic information. When a fault is developed in the normal optical path, extra-traffic path is interrupted and channels are released for use in the faulty path to continue operating the network.
According to the network described above, for a full mesh configuration, the number of waves to be selected within a node is much less than the necessary number of channels required within the network (by a factor of 1/N2 of the necessary number of waves) so that, the minimum number of waves to satisfy the customer needs can be selected from the optical path termination circuit while maintaining the performance level, thereby reducing the number of hardwares required within each node. Also, by using WDM transmission lines for transmitting multiple waves between the OADMs and optical path termination circuits, the number of optical signal lines between the OADMs and optical path termination circuits can be reduced.
Also, by establishing a full mesh configuration using WDM transmission lines between the nodes, not only the number of optical fibers needed for connection is reduced, but the wave utilization efficiency is increased in each node. Thus, the ring network can be operated at its optimum efficiency.
Network protection is based on switching an optical path containing a fault, therefore, problems inside the node (such as problems in optical path termination circuits or OADMs) can be resolved. The optical path length is also reduced so that ring network having a larger ring radius can be designed using lesser number of relay stations.
During the normal operation of the network, protection waves can be used to carry extra-traffic information, so that wave utilization efficiency is increased and also different qualities of services can be offered within one ring network.
A sixth aspect of the network is that by increasing the network capacity on the basis of 2F-BR architecture, the enlarged network can be operated to meet the increased demand by the separating the WDM transmission lines into emergency-waves for use only during the emergency and normal-use waves for use only during the normal operation so that the increased capacity network is operated as a 4F-BR ring network.
In this case, the waves processed by the OADMs are separated into normal-use waves and emergency-use waves, and these waves are transmitted through separate WDM transmission lines for processing by the respective OADMs.
Therefore, even when the network is operated in the 2F-BR mode, the separation of protection path means that the normal mode of operation is less likely to be affected because all the processing devices are separated.
Accordingly, network capacity increase can be provided economically by operating the system as a 2F-BR network initially, and gradually adding another 2F-BR network when all the waves are utilized.
Also, when increasing the capacity, the new network may be based only on emergency-use waves so that the WDM transmission lines are comprised by separate lines of normal-use waves and emergency-use waves. Therefore, the normal service can be continued if a fault is developed within the WDM line using the emergency-use waves or inside the node itself, normal services are not disrupted at all.
Connection switching is performed without stopping the operation of the normal-use optical paths
An advantage of this network is that the emergency waves may be used for information different from the normal information, i.e., to transmit extra-traffic information.
The extra-traffic information line can be stopped when adding a new service or if a fault is developed in the network, so that two different quality services can be offered thereby providing flexibility to the users.
As explained above, by means of the present invention, by means of making individual optical paths switching units, it is possible to reduce the transmission distance of an optical path during failure, and by means of a mechanism which carries out switching of optical path units, it is possible to respond to failure within nodes.
In addition, by means of the wave selection function provided in transmit ends, it is possible to efficiently utilize wave resources by means of reduction in the scale of couplers, reduction in modulators, addition of wave senders and wave selectors or tunable wave senders, and full mesh connection between nodes.
It is possible to reduce the scales of the optical couplers (multiplexers and de-multiplexers) and the number of modulators required. Increase in the number of wave senders and selectors as well as increase in the number of tunable wave senders are possible so as to connect in a full mesh configuration so that the resource utilization efficiency is significantly improved compared with the conventional network configurations.
All of the above aspects of the present invention contribute to reduction in the physical size of the optical path termination circuits so that a large-scale network can be designed economically and efficiently.
When the extra-traffic service is offered, two quality levels of communication services can be provided within one network.
Also, by separating the waves to be processed in the OADM into two groups, normal-use waves and emergency-use or extra-traffic waves, the maximum scale of wave couplers and multiplexers in the optical termination circuit can be halved.
By controlling such wave couplers and multiplexers separately as normal-use devices and emergency-use devices or extra-traffic devices, normal communication services will not be affected at all even if failures are experienced in any of the devices used for emergency or extra traffic.
When the network capacity is increased, it is only necessary to switch connections for the emergency-use lines or extra-traffic wave lines so that normal-use lines are not affected at all.
By carrying out such switching of connections to produce a 4F-BR network, normal-use wave group and emergency-use group or extra-traffic wave group are all contained in separate WDM transmission lines so that the normal operation of the network is not affected by disruptions caused by optical path failures, or problems in the WDM lines transmitting multiplexed optical signals due to emergency-use waves or extra-traffic waves.
Similarly, OADMs are also provided for separate wave groups for emergency-use and extra-traffic use so that, so long as separate input/output lines are provided for each wave group, switching can be performed on the basis of single waves or M-channels without limiting the structure of the optical path termination circuits.
The result is that a high-performance ring-network can be constructed economically and efficiently by reducing not only the size and scale but cost of manufacturing each component, but by improving the network reliability significantly. Furthermore, optical path termination devices can be modified and selected depending on the local conditions of customer usage, thereby enabling to alter the configuration of any ring network according to any changes in the level of demand for additional services.